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Clinical assessment of renal function
Renal
Clinical assessment of renal function
Learning objectives After reading this article, you should be able to: • take a complete history in a patient with renal dysfunction • identify five common causes of acute kidney injury in the hospital setting • describe three clinical signs of volume depletion and three of fluid overload • initiate appropriate investigations and interpret the results • state above what serum creatinine has been shown to affect adverse outcomes after general surgery.
Declan G de Freitas Michael Picton
Abstract The kidney carries out many key functions in the body, including the maintenance of fluid, acid–base and electrolyte homeostasis, the removal of nitrogenous waste, the production of erythropoietin, the hydroxylation of vitamin D and an important influence on regulation of blood pressure. This perhaps explains the increased risk of morbidity and mortality postoperatively in patients with acute or chronic renal impairment. Identification of these patients preoperatively allows the assessment of risk to both the kidneys and other organs. Protective measures can then be used in these patients with close monitoring to minimize poor outcomes. Clinical assessment of kidney function involves a thorough history and physical examination, with supplementary blood and radiological investigations.
disease, Alport’s syndrome, hypertension, diabetes or reflux nephropathy. An immunosuppressive drug history in patients with a trans planted organ or chronic kidney disease is important because patients taking steroids may need supplemental doses following surgery or if they have sepsis or may need conversion to intra venous formulations. Patients might have been taking immuno suppressant drugs in the past, such as cyclophosphamide, which remain relevant. This group of patients is also at increased risk of infections, both common and opportunistic. In patients on dialysis, it is important to clarify which type of dialysis they receive, how it is performed (e.g. with an arterio venous fistula), its adequacy, whether anticoagulation is used and when dialysis last occurred. Ideally, patients on maintenance haemodialysis should be dialysed the day before surgery, allow ing time for reversal of anticoagulation and maintenance of suit able biochemistry. Short-duration haemodialysis (2 hours) in the absence of anticoagulation can be carried out on the day of surgery in semi-emergency cases, but it can be problematic with clotting of dialysis access and only a partial reduction in uraemic toxin and potassium levels. Peritoneal dialysis can be carried out until patients undergo their operation, but their abdominal fluid should be drained out before surgery to improve lung function.
Keywords creatinine; estimated glomerular filtration rate; protein– creatinine ratio; ultrasound; urine dipstick … bones can break, muscles can atrophy, glands can loaf, even the brain can go to sleep without immediately endangering our survival; but should the kidneys fail … neither bone, muscle, gland nor brain could carry on Homer W Smith (1895–1962) From Fish to Philosopher, ch. 1
Patient history Physical examination
Patients with newly diagnosed kidney impairment should have a history taken to identify its cause and any potential complica tions. Systemic conditions such as diabetes mellitus, hypertension and systemic lupus erythematosus can all cause cardiovascular disease, the risk of which is increased in the presence of kid ney impairment. Common causes of acute kidney injury in the hospital setting that are potentially preventable include hypo volaemia, hypotension, sepsis, intravenous contrast and nephro toxic drugs. There might be a family history of polycystic kidney
Physical examination is helpful in determining the aetiology (e.g. rash and vasculitis) and complications of kidney disease (e.g. pul monary oedema). The two most important assessments in patients with kidney disease include an appraisal of volume status and look ing for evidence of uraemia. The signs of volume depletion include postural hypotension, decreased skin turgor and low urine output. The signs of fluid overload include hypertension, elevated jugular venous pressure and pulmonary or peripheral oedema. Patients on maintenance dialysis should know their ‘target weight’, which is their estimated euvolaemic weight. However, undiagnosed weight loss can lead to hypervolaemia despite patients being at their ‘target weight’. Uraemic signs which indicate the need for urgent dialysis include encephalopathy, asterixis and a pericardial friction rub.
Declan G de Freitas MRCPI is a Specialist Registrar in Renal Medicine and Transplantation at the Manchester Royal Infirmary, UK. His specialist interests include ischaemia–reperfusion injury and encapsulating peritoneal sclerosis. Conflicts of interest: none declared.
Investigations
Michael Picton FRCP PhD is a Consultant Nephrologist and Transplant Physician at the Manchester Royal Infirmary. His specialist interests include ischaemia–reperfusion injury and ABO-incompatible transplantation. Conflicts of interest: none declared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 10:6
Serum creatinine The most widely used clinical index of kidney function is the serum concentration of creatinine. Creatinine is a by-product 293
© 2009 Elsevier Ltd. All rights reserved.
Renal
of the metabolism of creatine in skeletal muscle. Production of creatinine is directly proportional to lean body mass and remains relatively constant. This results in stable serum concentrations, allowing a convenient, reproducible and low-cost assessment of kidney function, which can be serially analysed in the same patient to monitor function or to identify acute kidney injury. Serum creatinine measurement alone has major limitations as a marker of kidney function: • Reference ranges are often given by laboratories, but serum values need to be considered in relation to lean body size rather than reference ranges. For example, an 80 kg man and a 40 kg woman can have a serum creatinine of 88 μmol/l, which is in the normal range, but they can have a glomerular filtration rate (eGFR) of 111 ml/min and 42 ml/min respectively. • Rhabdomyolysis can elevate serum creatinine independently of changes in GFR, as can certain drugs such as cimetidine and trimethoprim which block tubular secretion of creatinine. • Serum creatinine is an insensitive index of renal function in the early and late stages of renal disease, as shown in Figure 1. The relationship between serum creatinine and GFR is nonlinear, with little or no change in serum creatinine until a ma jor decrease in GFR has occurred. There are two contributing mechanisms to this phenomenon: (1) in response to injury, noninjured nephrons undergo hypertrophy and hyperfiltration to compensate for the loss of functioning nephrons and (2) tubular secretion of creatinine, which normally contributes minimally to overall creatinine clearance, increases progressively as kidney function deteriorates. Serum creatinine can also be used as a marker for increased morbidity and mortality after surgery.2 A serum creatinine level >150 μmol/l has been shown to be associated with increased cardiac, respiratory, haematological and infectious complications as well as 30-day mortality.
Estimated glomerular filtration rate The GFR, defined as the volume of ultrafiltrate generated by the kidneys per unit time (ml/min), is the most useful measure of kidney function because it helps to predict, and correlates with, complications associated with kidney disease. Water-soluble drugs are excreted renally; thus, the dosage of these drugs needs to be adjusted in patients with a reduced GFR to prevent accumu lation and toxicity. Erythropoietin production begins to decline as GFR falls below 70 ml/min, resulting in an increased risk of anaemia. Cardiovascular risk also increases as the GFR declines. In an attempt to improve the accuracy of serum creatinine as a measure of GFR, a number of formulae have been devised to estimate GFR from serum creatinine, accounting for variations in sex, weight and age. The most widely used formulae include the Cockcroft–Gault formula and the modification of diet in renal disease (MDRD) equation. The Cockcroft–Gault formula was developed to allow estim ation of creatinine clearance and not GFR, but it requires the patient’s weight, which is not readily available to laboratories. It is, however, the most used formula for calculating drug dosing. The best validated method currently uses the MDRD study data, from which a four-variable equation was derived that includes age, sex, creatinine and race (black or not black).3 This equation is more precise than the Cockcroft–Gault formula, particularly at lower GFRs, and has the advantage of not requiring the patient’s weight. The Department of Health in England has endorsed the use of the MDRD formula by all clinical biochemistry laboratories, and most UK laboratories now report estimated GFR (eGFR) when returning creatinine values. This is in combination with harmo nization of the results of serum creatinine assays by the National External Quality Assurance Scheme, to allow comparability of GFR estimates between laboratories. As a result, these values should be more accurate than calculations performed independently because they should incorporate laboratory-specific correction factors. The formula has a number of limitations. Its confidence limits are wide: 90% of values are within 30% of the true value, which can result in misclassification and a diagnosis of chronic kidney disease. The formula has not been well validated in very old, very young or pregnant patients, or in ethnic minority groups other than African Americans. It has not been validated in acute kidney injury or malnutrition or in people at extremes of body size.
Relationship between serum creatinine and measured glomerular filtration rate in patients with renal failure 1000
Serum creatinine (µmol/l)
900
Creatinine clearance Measurement of creatinine clearance is dependent on an accurate 24-hour urine collection and a simultaneous serum creatinine measurement, which many patients find difficult and inconve nient to achieve. This has been superseded by eGFR measure ment in addition to spot urine protein–creatinine ratios, which can be extrapolated to 24 hour urinary protein excretion.
800 700 600 500 400 300 200 100
V
IV
III
II
CKD stage
0 0
20
40
60
Serum urea Urea is excreted primarily by the glomerulus, and serum levels of urea are inversely correlated with GFR. Urea is not, however, a reliable marker of kidney function. Urea production can depend on protein intake, liver function and catabolic rate. In addition, it is absorbed from the glomerular filtrate, particularly under conditions of low urine flow rate, such as volume depletion. In patients nearing end-stage renal failure, urea levels can be very high with no apparent symptoms (azotaemia) and a particular
80
100
120
Actual GFR (ml/min/1.73 m2) Adapted from Shemesh et al.1
Figure 1
ANAESTHESIA AND INTENSIVE CARE MEDICINE 10:6
294
© 2009 Elsevier Ltd. All rights reserved.
Renal
urea concentration in itself is not an indication for renal replace ment therapy, unlike uraemia.
e chogenicity can help to determine whether the kidney disease is acute or chronic. Hydronephrosis, perinephric collections, cystic diseases of the kidney and renal stones can also be diagnosed. Doppler ultrasound can be used to assess renal blood flow and indicate renal artery stenosis or vein thrombosis. The diameter of the inferior vena cava can also provide useful information on volume status and guide fluid replacement. Apart from diagnosing calcium-containing stones in the urin ary tract, plain radiographs of the kidneys, ureter and bladder offer little in the initial evaluation of kidney disease or function. Both computed tomography (CT) and magnetic resonance imaging (MRI) are useful in imaging solid renal masses, in addi tion to diagnosing renal artery stenosis. Both, however, require a contrast agent that can be nephrotoxic in the case of CT or precipitate nephrogenic systemic fibrosis in the case of MRI. Risk factors for contrast nephropathy include hypovolaemia, serum creatinine >150 μmol/l, contrast medium >2 ml/kg and age >60 years. Good hydration and N-acetylcysteine given before intravenous injection of contrast have been shown in some stud ies to be preventative. Radionuclide imaging is useful in the evaluation of reno vascular disease and kidney transplant function, as well as when a unilateral nephrectomy is considered, because it allows a more accurate determination of the GFR (calculated GFR).
Other laboratory investigations In the setting of a single serum creatinine measurement, other investigations can help differentiate acute from chronic kidney dis ease, although their specificity must be regarded as low. Anaemia, hypocalcaemia, hyperparathyroidism and hyperphosphataemia may suggest chronic kidney disease, but are often seen in acute renal failure and can also be explained by other disorders. Urine dipstick Urine dipstick testing is helpful, cheap and easy to perform. A positive dipstick for blood indicates the presence of erythrocytes, free haemoglobin or myoglobin in the urine. No visible erythro cytes on microscopy raises the possibility of haemoglobinuria or myoglobinuria. The presence of leucocyte esterases or nitrites in the urine indicates a possible urinary tract infection. Detection of glucose in the urine can indicate hyperglycaemia, although, in patients with diabetes, glucosuria can occur after the blood sugar has normalized. Dipstick testing can also detect the presence of ketones, indicating ketoacidosis secondary to alcohol, starvation or diabetes. Finally, proteinuria can be detected once values are greater than 300 mg/l (normal <150 mg/l), indicating tubular or glomerular disease. However, a positive result should be fol lowed with a urine albumin–creatinine ratio to accurately quan tify the presence of protein. Furthermore, urine dipstick is not reliable at detecting Bence Jones proteins in the urine.
Conclusion Chronic kidney disease or acute-on-chronic kidney injury is the most frequent scenario seen in the hospital setting. Most patients will already have been seen by a nephrologist, especially with the quality and outcomes framework (QOF) initiative among GPs, and thus will have had their kidney function assessed. However, reassessment before surgery can help to stratify the risk for patients. Newly diagnosed patients and those with acute injury require at least a basic assessment of their kidney func tion, including a thorough history and examination, an MDRD eGFR and renal ultrasound imaging. For patients with compli cations, it is worthwhile involving a nephrologist early in the assessment process to best manage risk and reduce postoperative complications. ◆
Urinanalysis Normal urine sediment is almost cell free and usually crystal free. The presence of erythrocytes (> 3 red blood cells per highpower field), leucocytes and crystals all indicate an underlying renal pathology. Urine protein–creatinine ratio and urine albumin–creatinine ratio The limitations of 24-hour urine collections for urinary protein estimation has led to their replacement by early morning urine protein–creatinine ratios, which have a good correlation with the 24-hour urine collection results. If the urine protein–creatinine level is expressed in mg/mg, the value obtained is approximately the same as g/24 hours of urinary protein excretion. If the ratio is expressed as mg/mmol, 24-hour protein excretion is approxi mately 10 times this figure (based on an assumed average uri nary creatinine excretion of 10 mmol/day). The decision about whether to measure protein or albumin is largely based on cost (albumin is more expensive to assay than total protein but is more accurate). The urine protein–creatinine ratio will generally be higher than the albumin–creatinine ratio owing to the measurement of proteins other than albumin. There is no simple method for extrapolating from one to the other, and, to detect microalbuminuria, an albumin–creatinine ratio is required.
References 1 Shemesh O, Golbetz H, Kriss JP, Myers BD. Limitations of creatinine as a filtration marker in glomerulopathic patients. Kidney Int 1985; 28: 830–8. 2 O’Brien MM, Gonzales R, Shroyer AL, et al. Modest serum creatinine elevation affects adverse outcome after general surgery. Kidney Int 2002; 62: 585–92. 3 Levey AS, Bosch JP, Lewis JB, et al. A more accurate method to estimate glomerular filtration rate from serum creatinine: a new prediction equation. Modification of Diet in Renal Disease Study Group. Ann Intern Med 1999; 130: 461–70. Further reading Renal Association. Guidelines on the detection and management of CKD. Also available at: www.renal.org/pages/pages/guidelines/ current/ckd.php
Imaging Renal ultrasound is very important in the initial evaluation of suspected kidney disease. Information on kidney size and
ANAESTHESIA AND INTENSIVE CARE MEDICINE 10:6
295
© 2009 Elsevier Ltd. All rights reserved.
Clinical assessment of renal function
Learning objectives After reading this article, you should be able to: • take a complete history in a patient with renal dysfunction • identify five common causes of acute kidney injury in the hospital setting • describe three clinical signs of volume depletion and three of fluid overload • initiate appropriate investigations and interpret the results • state above what serum creatinine has been shown to affect adverse outcomes after general surgery.
Declan G de Freitas Michael Picton
Abstract The kidney carries out many key functions in the body, including the maintenance of fluid, acid–base and electrolyte homeostasis, the removal of nitrogenous waste, the production of erythropoietin, the hydroxylation of vitamin D and an important influence on regulation of blood pressure. This perhaps explains the increased risk of morbidity and mortality postoperatively in patients with acute or chronic renal impairment. Identification of these patients preoperatively allows the assessment of risk to both the kidneys and other organs. Protective measures can then be used in these patients with close monitoring to minimize poor outcomes. Clinical assessment of kidney function involves a thorough history and physical examination, with supplementary blood and radiological investigations.
disease, Alport’s syndrome, hypertension, diabetes or reflux nephropathy. An immunosuppressive drug history in patients with a trans planted organ or chronic kidney disease is important because patients taking steroids may need supplemental doses following surgery or if they have sepsis or may need conversion to intra venous formulations. Patients might have been taking immuno suppressant drugs in the past, such as cyclophosphamide, which remain relevant. This group of patients is also at increased risk of infections, both common and opportunistic. In patients on dialysis, it is important to clarify which type of dialysis they receive, how it is performed (e.g. with an arterio venous fistula), its adequacy, whether anticoagulation is used and when dialysis last occurred. Ideally, patients on maintenance haemodialysis should be dialysed the day before surgery, allow ing time for reversal of anticoagulation and maintenance of suit able biochemistry. Short-duration haemodialysis (2 hours) in the absence of anticoagulation can be carried out on the day of surgery in semi-emergency cases, but it can be problematic with clotting of dialysis access and only a partial reduction in uraemic toxin and potassium levels. Peritoneal dialysis can be carried out until patients undergo their operation, but their abdominal fluid should be drained out before surgery to improve lung function.
Keywords creatinine; estimated glomerular filtration rate; protein– creatinine ratio; ultrasound; urine dipstick … bones can break, muscles can atrophy, glands can loaf, even the brain can go to sleep without immediately endangering our survival; but should the kidneys fail … neither bone, muscle, gland nor brain could carry on Homer W Smith (1895–1962) From Fish to Philosopher, ch. 1
Patient history Physical examination
Patients with newly diagnosed kidney impairment should have a history taken to identify its cause and any potential complica tions. Systemic conditions such as diabetes mellitus, hypertension and systemic lupus erythematosus can all cause cardiovascular disease, the risk of which is increased in the presence of kid ney impairment. Common causes of acute kidney injury in the hospital setting that are potentially preventable include hypo volaemia, hypotension, sepsis, intravenous contrast and nephro toxic drugs. There might be a family history of polycystic kidney
Physical examination is helpful in determining the aetiology (e.g. rash and vasculitis) and complications of kidney disease (e.g. pul monary oedema). The two most important assessments in patients with kidney disease include an appraisal of volume status and look ing for evidence of uraemia. The signs of volume depletion include postural hypotension, decreased skin turgor and low urine output. The signs of fluid overload include hypertension, elevated jugular venous pressure and pulmonary or peripheral oedema. Patients on maintenance dialysis should know their ‘target weight’, which is their estimated euvolaemic weight. However, undiagnosed weight loss can lead to hypervolaemia despite patients being at their ‘target weight’. Uraemic signs which indicate the need for urgent dialysis include encephalopathy, asterixis and a pericardial friction rub.
Declan G de Freitas MRCPI is a Specialist Registrar in Renal Medicine and Transplantation at the Manchester Royal Infirmary, UK. His specialist interests include ischaemia–reperfusion injury and encapsulating peritoneal sclerosis. Conflicts of interest: none declared.
Investigations
Michael Picton FRCP PhD is a Consultant Nephrologist and Transplant Physician at the Manchester Royal Infirmary. His specialist interests include ischaemia–reperfusion injury and ABO-incompatible transplantation. Conflicts of interest: none declared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 10:6
Serum creatinine The most widely used clinical index of kidney function is the serum concentration of creatinine. Creatinine is a by-product 293
© 2009 Elsevier Ltd. All rights reserved.
Renal
of the metabolism of creatine in skeletal muscle. Production of creatinine is directly proportional to lean body mass and remains relatively constant. This results in stable serum concentrations, allowing a convenient, reproducible and low-cost assessment of kidney function, which can be serially analysed in the same patient to monitor function or to identify acute kidney injury. Serum creatinine measurement alone has major limitations as a marker of kidney function: • Reference ranges are often given by laboratories, but serum values need to be considered in relation to lean body size rather than reference ranges. For example, an 80 kg man and a 40 kg woman can have a serum creatinine of 88 μmol/l, which is in the normal range, but they can have a glomerular filtration rate (eGFR) of 111 ml/min and 42 ml/min respectively. • Rhabdomyolysis can elevate serum creatinine independently of changes in GFR, as can certain drugs such as cimetidine and trimethoprim which block tubular secretion of creatinine. • Serum creatinine is an insensitive index of renal function in the early and late stages of renal disease, as shown in Figure 1. The relationship between serum creatinine and GFR is nonlinear, with little or no change in serum creatinine until a ma jor decrease in GFR has occurred. There are two contributing mechanisms to this phenomenon: (1) in response to injury, noninjured nephrons undergo hypertrophy and hyperfiltration to compensate for the loss of functioning nephrons and (2) tubular secretion of creatinine, which normally contributes minimally to overall creatinine clearance, increases progressively as kidney function deteriorates. Serum creatinine can also be used as a marker for increased morbidity and mortality after surgery.2 A serum creatinine level >150 μmol/l has been shown to be associated with increased cardiac, respiratory, haematological and infectious complications as well as 30-day mortality.
Estimated glomerular filtration rate The GFR, defined as the volume of ultrafiltrate generated by the kidneys per unit time (ml/min), is the most useful measure of kidney function because it helps to predict, and correlates with, complications associated with kidney disease. Water-soluble drugs are excreted renally; thus, the dosage of these drugs needs to be adjusted in patients with a reduced GFR to prevent accumu lation and toxicity. Erythropoietin production begins to decline as GFR falls below 70 ml/min, resulting in an increased risk of anaemia. Cardiovascular risk also increases as the GFR declines. In an attempt to improve the accuracy of serum creatinine as a measure of GFR, a number of formulae have been devised to estimate GFR from serum creatinine, accounting for variations in sex, weight and age. The most widely used formulae include the Cockcroft–Gault formula and the modification of diet in renal disease (MDRD) equation. The Cockcroft–Gault formula was developed to allow estim ation of creatinine clearance and not GFR, but it requires the patient’s weight, which is not readily available to laboratories. It is, however, the most used formula for calculating drug dosing. The best validated method currently uses the MDRD study data, from which a four-variable equation was derived that includes age, sex, creatinine and race (black or not black).3 This equation is more precise than the Cockcroft–Gault formula, particularly at lower GFRs, and has the advantage of not requiring the patient’s weight. The Department of Health in England has endorsed the use of the MDRD formula by all clinical biochemistry laboratories, and most UK laboratories now report estimated GFR (eGFR) when returning creatinine values. This is in combination with harmo nization of the results of serum creatinine assays by the National External Quality Assurance Scheme, to allow comparability of GFR estimates between laboratories. As a result, these values should be more accurate than calculations performed independently because they should incorporate laboratory-specific correction factors. The formula has a number of limitations. Its confidence limits are wide: 90% of values are within 30% of the true value, which can result in misclassification and a diagnosis of chronic kidney disease. The formula has not been well validated in very old, very young or pregnant patients, or in ethnic minority groups other than African Americans. It has not been validated in acute kidney injury or malnutrition or in people at extremes of body size.
Relationship between serum creatinine and measured glomerular filtration rate in patients with renal failure 1000
Serum creatinine (µmol/l)
900
Creatinine clearance Measurement of creatinine clearance is dependent on an accurate 24-hour urine collection and a simultaneous serum creatinine measurement, which many patients find difficult and inconve nient to achieve. This has been superseded by eGFR measure ment in addition to spot urine protein–creatinine ratios, which can be extrapolated to 24 hour urinary protein excretion.
800 700 600 500 400 300 200 100
V
IV
III
II
CKD stage
0 0
20
40
60
Serum urea Urea is excreted primarily by the glomerulus, and serum levels of urea are inversely correlated with GFR. Urea is not, however, a reliable marker of kidney function. Urea production can depend on protein intake, liver function and catabolic rate. In addition, it is absorbed from the glomerular filtrate, particularly under conditions of low urine flow rate, such as volume depletion. In patients nearing end-stage renal failure, urea levels can be very high with no apparent symptoms (azotaemia) and a particular
80
100
120
Actual GFR (ml/min/1.73 m2) Adapted from Shemesh et al.1
Figure 1
ANAESTHESIA AND INTENSIVE CARE MEDICINE 10:6
294
© 2009 Elsevier Ltd. All rights reserved.
Renal
urea concentration in itself is not an indication for renal replace ment therapy, unlike uraemia.
e chogenicity can help to determine whether the kidney disease is acute or chronic. Hydronephrosis, perinephric collections, cystic diseases of the kidney and renal stones can also be diagnosed. Doppler ultrasound can be used to assess renal blood flow and indicate renal artery stenosis or vein thrombosis. The diameter of the inferior vena cava can also provide useful information on volume status and guide fluid replacement. Apart from diagnosing calcium-containing stones in the urin ary tract, plain radiographs of the kidneys, ureter and bladder offer little in the initial evaluation of kidney disease or function. Both computed tomography (CT) and magnetic resonance imaging (MRI) are useful in imaging solid renal masses, in addi tion to diagnosing renal artery stenosis. Both, however, require a contrast agent that can be nephrotoxic in the case of CT or precipitate nephrogenic systemic fibrosis in the case of MRI. Risk factors for contrast nephropathy include hypovolaemia, serum creatinine >150 μmol/l, contrast medium >2 ml/kg and age >60 years. Good hydration and N-acetylcysteine given before intravenous injection of contrast have been shown in some stud ies to be preventative. Radionuclide imaging is useful in the evaluation of reno vascular disease and kidney transplant function, as well as when a unilateral nephrectomy is considered, because it allows a more accurate determination of the GFR (calculated GFR).
Other laboratory investigations In the setting of a single serum creatinine measurement, other investigations can help differentiate acute from chronic kidney dis ease, although their specificity must be regarded as low. Anaemia, hypocalcaemia, hyperparathyroidism and hyperphosphataemia may suggest chronic kidney disease, but are often seen in acute renal failure and can also be explained by other disorders. Urine dipstick Urine dipstick testing is helpful, cheap and easy to perform. A positive dipstick for blood indicates the presence of erythrocytes, free haemoglobin or myoglobin in the urine. No visible erythro cytes on microscopy raises the possibility of haemoglobinuria or myoglobinuria. The presence of leucocyte esterases or nitrites in the urine indicates a possible urinary tract infection. Detection of glucose in the urine can indicate hyperglycaemia, although, in patients with diabetes, glucosuria can occur after the blood sugar has normalized. Dipstick testing can also detect the presence of ketones, indicating ketoacidosis secondary to alcohol, starvation or diabetes. Finally, proteinuria can be detected once values are greater than 300 mg/l (normal <150 mg/l), indicating tubular or glomerular disease. However, a positive result should be fol lowed with a urine albumin–creatinine ratio to accurately quan tify the presence of protein. Furthermore, urine dipstick is not reliable at detecting Bence Jones proteins in the urine.
Conclusion Chronic kidney disease or acute-on-chronic kidney injury is the most frequent scenario seen in the hospital setting. Most patients will already have been seen by a nephrologist, especially with the quality and outcomes framework (QOF) initiative among GPs, and thus will have had their kidney function assessed. However, reassessment before surgery can help to stratify the risk for patients. Newly diagnosed patients and those with acute injury require at least a basic assessment of their kidney func tion, including a thorough history and examination, an MDRD eGFR and renal ultrasound imaging. For patients with compli cations, it is worthwhile involving a nephrologist early in the assessment process to best manage risk and reduce postoperative complications. ◆
Urinanalysis Normal urine sediment is almost cell free and usually crystal free. The presence of erythrocytes (> 3 red blood cells per highpower field), leucocytes and crystals all indicate an underlying renal pathology. Urine protein–creatinine ratio and urine albumin–creatinine ratio The limitations of 24-hour urine collections for urinary protein estimation has led to their replacement by early morning urine protein–creatinine ratios, which have a good correlation with the 24-hour urine collection results. If the urine protein–creatinine level is expressed in mg/mg, the value obtained is approximately the same as g/24 hours of urinary protein excretion. If the ratio is expressed as mg/mmol, 24-hour protein excretion is approxi mately 10 times this figure (based on an assumed average uri nary creatinine excretion of 10 mmol/day). The decision about whether to measure protein or albumin is largely based on cost (albumin is more expensive to assay than total protein but is more accurate). The urine protein–creatinine ratio will generally be higher than the albumin–creatinine ratio owing to the measurement of proteins other than albumin. There is no simple method for extrapolating from one to the other, and, to detect microalbuminuria, an albumin–creatinine ratio is required.
References 1 Shemesh O, Golbetz H, Kriss JP, Myers BD. Limitations of creatinine as a filtration marker in glomerulopathic patients. Kidney Int 1985; 28: 830–8. 2 O’Brien MM, Gonzales R, Shroyer AL, et al. Modest serum creatinine elevation affects adverse outcome after general surgery. Kidney Int 2002; 62: 585–92. 3 Levey AS, Bosch JP, Lewis JB, et al. A more accurate method to estimate glomerular filtration rate from serum creatinine: a new prediction equation. Modification of Diet in Renal Disease Study Group. Ann Intern Med 1999; 130: 461–70. Further reading Renal Association. Guidelines on the detection and management of CKD. Also available at: www.renal.org/pages/pages/guidelines/ current/ckd.php
Imaging Renal ultrasound is very important in the initial evaluation of suspected kidney disease. Information on kidney size and
ANAESTHESIA AND INTENSIVE CARE MEDICINE 10:6
295
© 2009 Elsevier Ltd. All rights reserved.